In a liquid crystal display element such as a twisted nematic, super twisted nematic, or active matrix liquid crystal display element, specifically a medium or small sized liquid crystal display element for a cellular phone, display stability to environmental temperature during use is an essential factor, and liquid crystal materials has been desired with such a low driving voltage that electrical power consumption can be reduced while maintaining responsiveness, with a small frequency dependence of duty driving or a small dependence of an optical property on a driving voltage in a temperature range of −30 to 0° C. or 40 to 80° C. or on a steep driving voltage, and so on.
Specifically, a temperature range of a liquid crystalline phase, which is an absolute value of a difference between a lower temperature limit (T→N) and an upper temperature limit (TN-I) of a liquid crystalline phase, is essential since outdoor use has become common because of the low electrical power consumption of a liquid crystal display element. Also, the demand for a reflective or semi-transmissive liquid crystal display element which does not require a backlight has increased for low electrical power consumption, and a liquid crystal material with a low birefringence has been desired to improve display characteristics.
In the present situation, however, a sufficiently low birefringence value required for a reflective or semi-transmissive liquid crystal display element using a combination of conventional materials has not been achieved, while it is possible to increase the temperature range of a liquid crystalline phase (see Patent Document 1). Therefore, the development of a liquid crystal composition with a sufficiently low birefringence without deteriorating the liquid crystal characteristics such as viscosity or threshold voltage, or the development of the best liquid crystal material for this purpose has been desired.
Meanwhile, a liquid crystal compound containing a trifluoromethoxy group is already known, and several compounds thereof and a liquid crystal composition using these compounds were disclosed (see Patent Documents 2 and 3). These Patent Documents 2 and 3 mention that it is possible for these compounds to be used as a nematic liquid crystal with an excellent co-solubility to liquid crystal compounds or compositions, a low viscosity enabling a quick response, a low threshold voltage enabling low voltage driving, a small birefringence, and a high voltage-holding ratio well enabling active matrix driving. However, Patent Documents 2 and 3 do not mention the specific constitutions for realizing a liquid crystal composition with an extremely small birefringence required for a reflective or semi-transmissive liquid crystal display element while sufficiently maintaining other liquid crystal characteristics.
Also, a liquid crystal composition with a small birefringence using a liquid crystal compound containing a trifluoromethoxy group is already known, and a preferable example thereof was disclosed (see Patent Document 4). However, since a lower temperature limit of a liquid crystalline phase is high, this composition cannot be used as a practical liquid crystal composition.
Therefore, the development of a nematic liquid crystal composition is desired with a small birefringence which is effective in a reflective or semi-transmissive liquid crystal display element and has excellent properties.
Patent Document 1: German Patent Application, Publication-No. 4023107 (see claim 1 on page 2)
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2002-53861 (see page 8 to examples)
Patent Document 3: Japanese Unexamined Patent Application, First Publication No. Hei 10-218825 (see page 8 to examples)
Patent Document 4: Published Japanese Translation No. 2002-533526 of PCT (see example 7 on page 32)
An object of the present invention is to provide a nematic liquid crystal composition with a wide temperature range of a liquid crystalline phase, a low threshold voltage, a low viscosity, and a small birefringence. Also, another object of the present invention is to provide a display element using this liquid crystal composition as a constituent member, specifically having excellent quality for a reflective or semi-transmissive liquid crystal display element.